Numerical modelling of soil nails in loose fill slope under surcharge loading

A plane–strain numerical model has been developed to mimic a nailed loose fill slope under surcharge loading. The model has been used to back-analyse a field test that was conducted to examine the behaviour of soil nails in loose fill slopes under surcharge loading. Incremental elasto-plastic analyses coupled with pore water diffusion have been performed to study the internal deformation, water content redistribution in the soil, and the performance of the soil nails during and after the application of surcharge loading. The model parameters describing the mechanical and hydraulic properties of the nailed slope were obtained from field or laboratory tests. Different modelling techniques and boundary conditions for mimicking soil–nail interaction in loose fill material have been examined. Comparisons between numerical predictions and field measurements demonstrate that a new interfacial model, denoted as the embedded bond–slip interface model, is more suitable for mimicking the interfacial behaviour. Despite the simplicity of the numerical model, the predicted responses are in close agreement with the field test results, in particular the mobilisation and distribution of nail forces in response to surcharge loading. Both the numerical and the field test results suggest that soil nails are capable of increasing the overall stability of a loose fill slope for the loading conditions considered in this study. The increase in confining stress along the soil nails near the surcharge area is central to the overall stabilising mechanism. On the contrary, the nail forces mobilised near the nail heads are much smaller, indicating that the beneficial effect of having a structural grillage system at the slope face is limited for the range of surcharge pressures considered in this study.     

降雨入渗对路堤边坡稳定性研究

基于非饱和土单变量的有效应力理论,建立了非饱和水-气两相的流体-固体耦合方程,在考虑地表径流-地下渗流和流体-固体双耦合的情况下,以达成线K240+10~70段因降雨产生的边坡破坏为工程实例,对计算理论模型进行了数值验证,同时对各种影响边坡稳定性因素进行了数值计算,对其破坏范围、降雨引起的破坏深度和入渗深度进行了分析,综合各种影响因素,其中降雨强度和历时及长期强度的影响最为明显。     

The mechanism of soil failures along cracks subjected to water infiltration

A field irrigation test was conducted at a crack-developed loess slope in Heifangtai, China, to study the slope instability that was caused by water infiltration. In the field test, the formation of sinkholes was considered a major type of soil failure. To investigate the mechanism of soil failures along cracks caused by water infiltration, a mechanical-hydraulic coupled analysis was conducted using a finite difference program. A moving boundary algorithm was proposed and adopted in the numerical model. Different draining rates of water in the crack were studied. The failure mechanism was identified as: the wetting-induced loss in shear strength of the soil, and the rapid loss of lateral support provided by the water during the dropping stage of water level. Under such conditions, a sinkhole is likely to form in the field.     

考虑孔隙水压力的土坡稳定性的有限元下限分析

以极限分析下限法理论为基础,应用有限单元思想离散结构物,建立了同时满足平衡条件、应力边界条件、屈服条件和应力间断条件的静力许可应力场,其中孔隙水压力被当作一种类似于重力的外力荷载。引入线性数学规划手段后,得到了考虑孔隙水压力的边坡稳定的下限法数学规划模型,由此可以求出安全系数的下限解及其对应的应力场。最后,以2个经典的土坡为算例,与多种方法的分析结果比较,论证了该方法的正确性。     

深圳某填土滑坡破坏机理研究

2002年9月18日13:40,在连降暴雨的影响下,深圳一填土边坡发生滑坡,约2.5×104m3的松散填土体在水平地面高速滑动了140m,造成5人死亡3、1人受伤。文章首先介绍了滑坡区的地质环境特点及滑坡特征;为了揭示该滑坡的发生机理,开展了松散击实填土的等压固结不排水剪(ICU)及常剪应力排水剪(CS)试验。ICU试验结果表明,土体具有明显的应变软化特性并伴随孔隙水压力的上升;CS试验结果表明,土体在低围压条件下破坏具有突然性,破坏过程中孔隙水压力急剧上升,表明土体出现突然性结构丧失。ICU及CS试验均表明松散填土具有静态液化特性。从现场孔隙比与稳态线(SSL)的关系来看,原位填土的状态参数位于稳态线上方,因此该填土边坡是由于地下水位上升造成土体出现静态液化、液化土体形成流滑所致。无限斜坡稳定分析表明造成该填土边坡破坏需要有较高的地下水位。     

考虑孔隙水压力的土坡稳定性的有限元上限分析

基于塑性极限分析的上限定理,考虑孔隙水压力的影响,以结点速度为未知量,根据塑性流动约束条件、速度间断约束条件、速度边界约束条件以及由外功功率与内功功率相等条件得到的目标函数,建立了求解孔隙水压力条件下土坡稳定性的有限元塑性极限分析线性规划数学模型;可求得边坡安全系数的上限值及相应的机动容许位移场;最后对2个经典算例进行了分析,验证了提出方法和程序是正确可行的。     

220kV盐津变电所填土边坡稳定性研究

针对该填土边坡,介绍了边坡的地质环境背景,描述了该边坡的变形特征,并从地形地貌、地层岩性、地质构造、水文地质条件等场地条件和人类活动等方面对边坡的变形机制进行了分析.结果表明:边坡变形模式是一种推移式蠕滑变形,场地平整填方加载是造成边坡变形的诱发因素,不良的地形地貌、地质结构和地下水条件是导致边坡蠕滑变形的内在因素.基于饱和三轴崮结不排水剪(CU)和排水剪(CD)试验、直剪与反复直剪试验以及岩块单轴抗压强度试验,通过反分析合理地确定了计算参数,通过有限差分软件FLAC3D如对填土边坡进行了变形破坏分析和加固后的稳定性评价.研究结果表明:天然状态下安全系数为1.10:填方堆载后其安全系数为1.03,坡体接近临界状态:抗滑加固后边坡的安全系数为1.27.因此,加固后边坡处于稳定状态.     

Simulation of a slope failure induced by rainfall infiltration

On 1 November 2000, Typhoon Xangsane brought heavy rainfall that caused serious disasters in many areas of northern Taiwan. A slope located at the upstream of the Shanher Stream in Taipei County failed and induced a debris flow. To investigate the environmental influencing factors that caused the slope failure and the debris flow disaster, laboratory tests and slope stability analysis were performed. A series of tests were conducted to obtain the mechanical and hydraulic properties of the soil under unsaturated and saturated conditions. Then, limit equilibrium method and numerical analysis simulating the process of infiltration were utilized to explore the slope stability, the stress variation, and the pore-water pressure in the soil during rainwater infiltration. The results of the stability analysis show that the rainfall intensity–time history is the most significant influence factor, and the analyzed failure zone and the predicted time when the slope failed are comparable to the field observation.     

预加固高填方边坡滑动破坏的离心模型试验研究

2009年10月3日,攀枝花机场260×104 m3万方的高填方预加固边坡整体失稳,沿基覆界面下滑,并超覆于其下方的易家坪老滑坡之上,使易家坪老滑坡再次复活。这一填方边坡失稳事件最终导致机场停航两年。通过大型离心模型试验,再现了攀枝花机场预加固高填方边坡的滑动失稳过程,获得了边坡变形破裂的特征参量,阐明了边坡的滑动失稳机制。研究表明：天然工况下,边坡变形以坡顶沉降和沿基覆界面软弱层的蠕滑为主,坡体总体处于蠕滑变形状态;降雨和地下水工况下,坡体中后部拉张破裂和中前部挤压变形显著,具有塑性流动及推移式破坏特征,并最终产生溃滑失稳;预加固抗滑桩承受较大的边坡变形推力,且越靠近坡体后部的桩推力越大,最终从后向前逐排产生累进性剪断破坏,说明该填方边坡多排抗滑桩的平面布置和受力确定欠合理;降雨和地下水工况下,坡体的最大孔隙水压力是天然工况下的3.7倍,孔隙水压力对坡体失稳有重要作用;通过模型和原型的综合对比分析,该边坡的滑动失稳机制为推移式蠕滑-累进性折断-溃滑与超覆。     

Dynamic behavior of railway embankment slope subjected to seismic excitation

To reveal the dynamic behavior of a railway embankment slope subjected to seismic excitation, a shaking table model test was performed on a 1:8 scale embankment slope. Different types of seismic wave of differing amplitudes were applied to study the dynamic behavior of the embankment slope, and white noise excitations were interspersed among the seismic waves to observe the changes of dynamic characteristics of the embankment slope. Residual deformation behaviors of the embankment slope were also investigated. The results of the tests show that the natural frequency of the embankment slope exhibits a decreasing trend and that the damping ratio exhibits an increasing trend. The embankment slope exhibits a significant amplification effect on the input acceleration, and the acceleration response differs greatly when subjected to different seismic excitations of differing spectrum characteristics. The filler of the embankment slope affects the changes of the spectrum characteristics of the seismic wave. The filler performs a filtering effect on high-frequency seismic waves and amplifies the energy of low-frequency seismic waves, especially when the frequency is close to the natural frequency of the embankment slope. A bidirectional excitation creates a greater acceleration response than a unidirectional excitation does. The seismic residual deformation of the embankment slope occurs under the seismic subsidence mode.     

Centrifuge modeling of the geogrid-reinforced slope subjected to differential settlement

A series of centrifuge model tests of geogrid-reinforced slopes with superstructure was conducted under differential settlement condition. The influence of reinforcement placement on the deformation and failure behavior of the slope and superstructure is investigated by considering different numbers of geogrid layers. The response of the slope and superstructure is analyzed based on a full-field displacement measurement via image analysis. The differential settlement induces distinguishable superstructure movement and slope deformation above the subsidence zone. The slope displacement, close to the vertical direction, appears only in a limited zone. An integrated analysis scheme of deformation and failure processes is adopted to reveal the failure mechanism of both the unreinforced and reinforced slopes: a certain level of deformation localization induced by differential settlement results in the local failure, and adversely, the local failure aggravates the deformation localization near it. The geogrid reinforcement mechanism is further clarified as the reduction on the deformation localization of the slope due to geogrid placement. The geogrid reinforcement effect can be comprehensively described with two respects: hooping effect and shielding effect, which illustrates the influence sphere and degree of geogrid reinforcement on restraining the slope deformation. The geogrid reinforcement is proven effective to improve the safety of the slope and superstructure.

     

坡面松散砾石土侵蚀过程及其特征研究

砾石土因其含砾、粉细砂和粘性土,具有级配宽、不均匀系数大、透水性强等特点,在土石坝防渗体土料和工程防渗墙中得到广泛应用。目前的研究多针对砾石土在工程应用方面,对于坡面松散砾石土降雨淋滤作用导致土体细粒成分流失、土体组构改变以至影响土体力学性质变化以及坡面物质稳定性变化等系列研究并不多见。利用人工降雨装置,进行了人工降雨坡面松散砾石土降雨淋滤试验,开展了砾石土侵蚀过程及其特征初步试验研究。试验表明,降雨作用下砾石土斜坡表面土体形态变化具有明显的阶段性特征,随着坡长的增加,产流率和侵蚀模数成线性增加,径流量随降雨时间跳跃和波动性增加,径流泥沙含量随降雨时间的增加以近似幂函数曲线关系递减。     

非均质土坡降雨入渗的耦合过程及稳定性分析

针对天然土质边坡通常呈现非均质特性这一问题,进行了降雨入渗作用下非均质土坡稳定性分析,考虑了渗流场和应力场之间的耦合作用。以一简化非均质土坡模型为例,对比了耦合和非耦合情况下渗流场的差异,验证耦合的客观性和其必要性;细-粗型和粗-细型土坡的降雨入渗过程可分别分为2个和3个阶段;运用有限元强度折减法,以特征部位位移突变并结合塑性区贯通作为失稳判据,进行了土坡稳定性分析,发现雨水到达土坡土层接触面的时刻即为边坡稳定性突变的时刻;并通过深入分析孔隙水压力分布,揭示了降雨诱发非均质土坡的变形破坏机制。同时,就降雨强度、历时和渗透系数对非均质土坡入渗过程和稳定性的影响进行了大量的计算和分析,发现只有当雨水进入到下层土体时,该层土体渗透系数才会对土坡稳定性产生较大影响。     

斜椭圆应力路径下饱和松砂动力特性试验研究

利用空心圆柱扭剪仪模拟斜入射地震波作用形成的斜椭圆应力路径,对比研究了等向固结条件下饱和福建标准松砂在循环斜椭圆、圆形、扭剪、三轴路径下的动力特性。试验研究表明：土体循环孔压发展存在陡升型和陡降型两种模式;圆形路径下累积孔压增长速率最快,循环扭剪最小;归一化孔压与斜椭圆的倾角无关,但受斜椭圆长短轴比及动应力比影响。砂土的不排水动强度与动应力路径密切相关,循环扭剪和循环三轴最大,循环斜椭圆次之,圆形路径最小。地震波从特定角度入射时,形成近似圆形路径,若只将地震波视为垂直入射的S波,将高估地基土体抗液化强度。     

Stress–strain behaviour of a loosely compacted volcanic-derived soil and its significance to rainfall-induced fill slope failures

The stress–strain behaviour of a soil is crucial to the recognition of the mechanism of slope failure. Triaxial tests, composed of isotropically consolidated drained (ICD) and undrained compression (ICU) tests and anisotropically consolidated constant deviatoric stress path (CQD) tests, were carried out with the aim of investigating the stress–strain behaviour of loosely compacted volcanic-derived soils. The fact that the critical states are the same for ICU, ICD and CQD tests may show that the critical state is independent of the above three stress paths. The critical state line, as defined in critical state soil mechanics, is obtained from the e–log p′ and q–p′ plots based on the results of the above tests. The initial state of the consolidated specimens at initiation of failure may be classified as dilative or contractive in the light of the locations of the soil state relative to the critical state line. For contractional soils, the increased pore water pressure generated by rainfall infiltration leads to a contractive failure in a drained manner, giving rise to high excess pore water pressure. The excess pore water pressure caused by contraction cannot be dissipated instantly, resulting in a decrease in the shear resistance of the soil. The failure process is rapid. The failed soil mass is prone to flow after failure under the action of gravity due to its high moisture content and inflow of surface runoff and rainwater. For dilational soils, the increased pore water pressure resulting from infiltration leads to dilation, which reduces pore water pressure and thus increases the shear resistance of the soil. However, continued rainfall infiltration may be able to equilibrate the reduction in pore water pressure caused by dilation and, therefore, the dilation or displacement can continue. In Hong Kong, volcanic-derived soil is characterized by high permeability. Both the high permeability of volcanic-derived soil and a shallow failure surface make it possible for the reduction in pore pressure to equilibrate relatively quickly. Therefore, the failure is also rapid, at least for poorly compacted fill slopes.     

对水致黄土斜坡破坏模式及稳定性分析原则的思考

水对斜坡作用包括地表水流动作用和地表水入渗作用,地表水流动作用,如水库、河流的岸坡破坏,由水动力侵蚀所引起。目前黄土中地表水入渗影响下的斜坡稳定性分析存在一些概念含糊的问题,如忽略了入渗过程的应力路径,只考虑其破坏时的应力状态,这会导致对其破机理和稳定性计算参数取值的误判,文章只针对该类问题进行辨析讨论。黄土中地表水的入渗一般有降雨和灌溉两种,伴随降雨入渗多引起斜坡浅层破坏;灌溉导致地下水位上升则引起深层滑移。地表水入渗对斜坡总应力改变不大,水致斜坡破坏主要是孔隙水压力上升,土体有效应力降低所致。非饱和黄土中的初始孔隙水压力为负值,降雨入渗后的浅层黄土仍处于非饱和状态,孔压最大升到0;灌溉会引起地下水位抬升,潜水位下为正的孔隙水压力。明确了孔压变化过程,就可以用有效强度评价边坡稳定性。同时,目前一些观点认为关于流动性黄土滑坡是静态引起,这颠倒了因果关系,是滑移引起了液化,而不是液化导致的滑移。     

Analysis of biological clogging potential in a simulated compacted clay liner subjected to high-strength leachate infiltration

A bench scale study was conducted to assess the possibility and extent of biological clogging in compacted clayey soil exposed to high-strength leachate simulating conditions in a landfill. In two series of experiments, distilled–deionized water, slightly acidic water and fresh high-strength leachate were permeated through compacted clayey soil and the rate of infiltration was recorded. Colony-forming units per unit mass of soil were counted, and scanning electron microscope photographs were taken before and after termination of experiments. Results indicated that infiltration of leachate containing a very high concentration of organic matter followed a logistic fit indicating hydraulic clogging of the porous media. This was in agreement with a five order of magnitude greater bacterial growth compared to the original state of the soil and to cases where distilled–deionized and acidified water was used as the permeant. Water and acidified water infiltration followed a power fit indicating persistent infiltration through the end of experiments with no sign of clogging. Bacterial counts in these cases were similar to that of the original state of the soil. Photographs taken by scanning electron microscope also indicated formation of plate-like material within the soil texture in contrast to no change when water was used as the permeating fluid and to formation of holes when acidified water was infiltrated through the soil. It was concluded that biological clogging considerably reduced the rate of infiltration within compacted clayey soil shortly after exposure to high-strength leachate.     

An analytical solution for rainfall infiltration into an unsaturated infinite slope and its application to slope stability analysis

Surficial slope failures in residual soils are common in tropical and subtropical regions as a result of rainfall infiltration. This study develops an analytical solution for simulating rainfall infiltration into an infinite unsaturated soil slope. The analytical solution is based on the general partial differential equation for water flow through unsaturated soils. It can accept soil–water characteristic curve and unsaturated permeability function of the exponential form into account. Numerical simulations are conducted to verify the assumptions of the analytical solution and demonstrate that the proposed analytical solution is acceptable for the coarse soils with low air entry values. The pore‐water pressure (pwp) distributions obtained from the analytical solution can be incorporated into a limit equilibrium method to do infinite slope stability analysis for a rain‐induced shallow slip. The analysis method takes into account the influence of the water content change on unit weight and hence on factor of safety. A series of analytical parametric analyses have been performed using the developed model. The analyses indicate that when the residual soil slope, consisting of a completely decomposed granite layer underlain by a less permeable layer, is subjected to a continuous heavy rainfall, the loss of negative pwp and the reduction in factor of safety were found to be most significant for the shallow soil layer and during the first 12 h. The antecedent and subsequent rainfall intensity, depth of a less permeable layer and slope angle all have a significant influence on the pwp response and hence the slope stability. Copyright © 2012 John Wiley & Sons, Ltd.     

降雨条件下边坡水文响应及其变形过程分析——以深圳大龙山边坡为例

以深圳大龙山边坡为例,采用水分计、张力计、渗压计、固定式测斜仪和雨量计等针对边坡开展了降雨量、不同部位边坡土体基质吸力、体积含水量、孔隙水压力和深部水平变形等的监测,对降雨条件下边坡水文过程和内部变形进行了分析,发现当日降雨量较小时,如在2009年4月18日日降雨量为52.4mm/d的情况下,地表以下0.5m和1m深度土体含水量和基质吸力都发生响应;日降雨量较大时,如在2009年5月23日、24日连续两天日降雨量超过100mm/d的降雨情况下,3m深度内的体积含水量和基质吸力均表现响应;渗压计P4的地下水位距离地表只有约6m的距离,监测到瞬态孔隙水压力对降雨响应及其敏感,其余渗压计的孔隙水压力基本未发生变化,推测持时短的暴雨易造成山体边坡的浅层滑坡。同时,该边坡在3m内的累积位移相对较大,最大累积如测斜IN3A向累积位移接近10mm。但累积位移无继续增大的趋势,处于稳定状态。     

Continuum deformation and stability analyses of a steep hillside slope under rainfall infiltration

Rainfall weakens an earth slope in a number of ways. It increases the degree of saturation of the soil, thereby breaking the bonds created by surface tension between the soil particles. When the volume of infiltrating water is large enough to mobilize fluid flow inside the soil matrix, the fluid exerts a downhill frictional drag on the slope, creating a destabilizing effect. When excess fluid can no longer infiltrate the slope due to increased saturation in the soil, it is discharged as a surface runoff and erodes the slope. In this paper, we present a physics-based framework for continuum modeling of a hydrologically driven slope failure similar to what occurred in a steep experimental catchment CB1 near Coos Bay, Oregon. We quantify the rainfall-induced slope deformation and assess the failure potential of the slope using finite element modeling that couples solid deformation with fluid pressure in an unsaturated soil. Results of the studies suggest that for a steep hillside slope underlain by a shallow bedrock similar to the CB1 site, failure would occur by multiple slide blocks with the failure surfaces emerging on the slope face. These results suggest that an infinite slope mechanism would be insufficient to represent the failure kinematics for a slope similar to CB1.